Significant rework of the hardware picking code.

Previously each mapper had to make sure that the rendered
colors were correct for point/cell etc ID during hardware picking
and this required large datastructures and texture uploads
to the GPU on each pick.

Now it collects the color buffers and gives the mappers a chance to
update them. This allows us to use gl_VertexId and gl_PrimitiveId
directly in the shader and then if picked, allow the mapper to
adjust the color buffer as needed. This allows us to avoid
rebuilding the VBO and textures each time and avoids the memory
footprint related to that.